Turbine exhaust silencer



Nov. 22, 1966 L. s. WlRT TURBINE EXHAUST SILENCER Filed July 1, 1965 2 Sheets-Sheet 1 INVENTOR. (562/.5 .5. M67

Nov.-22, 1966 L. s. WlRT 3,236,787

TURBINE EXHAUST SILENCER Filed July 1, 1965 2 Sheets-Sheet 2 Ma y 11mm ll INVENTOR, 4652/5 6? M67 United States Patent 3,286,787 TURBINE EXHAUST SILENCER Leslie S. Wirt, Phoenix, Ariz., assignor to The Garrett Corporation, Los Angeles, Calitl, a corporation of California Filed July 1, 1965, Ser. No. 468,777 22 Claims. (Cl. 18167) This application is a continuation-impart of application Serial No. 304,314, filed August 26, 1963, entitled, Turbine Exhaust Silencer, now abandoned. The present invention relates generally to acoustical silencers, and more especially to silencers tor the exhaust of gas turbines and the like.

The exhaust from a gas turbine comprises a stream of hot gases moving at a high velocity. Discharge of this stream directly into the surrounding atmosphere creates a noise problem because the level of acoustic energy is substantially above the level at which people can work without damage to their ears, apart lirom the general discomfort and other problems attendant upon the existence of a noise source of this magnitude. Discharge of the exhaust stream poses the problem of reducing the level of sound created by this stream to as low -a level as is reasonably possible and, in any event, at least to a level at which ear damage to personnel nearby does not occur. Solution of this problem is complicated by the requirement, general, that a suitable silencer be of minimum bulk and therefore of high effectiveness.

The sound problem is particularly acute in the case of gas turbines on vehicles where the comfort of passengers must be taken into consideration as well as the comfort and safety of maintenance personnel. The design of an adequate silencer for installation on vehicle engines is made more difficult by the limitations on space and weight imposed on such equipment. On the other hand, a silencer suitable for use on a vehicle has the advantage that it has physical characteristics, such as size and weight, which make the unit readily adaptable to any stationary installation or to a portable installation tor ground test equipment and the like.

An important sourceof the sound waves in the exhaust stream is the combustion process carried on inside the turbine. In addition, turbulence in the exhaust duct itself is an important source of exhaust noise. The sound generated is mainly of a random character which changes somewhat in intensity with a change in the loading on the turbine, and which is usually most intense at frequencies above 160 cycles per second, typically having a peak intensity at around 6,000 cycles per second.

Thus it is a general object of the present invention to provide a silencer for this type of use that is light in weight and compact in size, in order o meet the requirements in this respect for a vehicular unit and is at the same time highly eifec-tive in reducing the nearby sound level. If these stringent requirements are met, the same silencer can also be easily adapted to installations where portability is important, such as a unit for test stands.

A further object of the invention is to provide -a silencer that is adapted to continuous operation at the advanced temperatures normally encountered with this type of use, operation of the silence-r with gas stream temperatures of 12001800 F. being contemplated.

Another object is to provide a silencer of 1 is character so designed that there is little or no possibility of tire burning within the silencer as a result of raw lfuel passing through the turbine and into the silencer during the start-up operation.

' Another object is to provide a silencer for use of this kind which avoids incorporation in the silencer of bulk ICC fibrous materials which have a relatively short life under the rigorous operating conditions imposed upon such ma.- terials when silencing a turbine exhaust.

Still another object of the present invention is to provide a silencer that is self-cooling, using ambient air, to reduce as far as possible the temperatures of the component parts of the silencer and especially the outer wall of the unit, thus reducing or eliminating the possibility of injury to persons or other objects coming into contact With the exterior of the silencer.

These objects of the present invention have been achieved in a silencer embodying the present invention by providing wall means of tubular shape forming a duct, having gas inlet and gas outlet openings through which the stream of exhaust gas llows, said wall means comprising a plurality of spaced layers of which at least the inner layer, and preferably more, is a metallic sheet laminar sound absorber having air paths through the layer permitting airborne sound waves to enter and pass through the layer with diminished intensity. Means are preferably provided to cause ambient air to flow through the annular space between the spaced layers of the wall means in order to remove heat therefrom by contact with the inner layer, aspirator means being used to induce such air flow between the layers and thence into the exhaust stream.

Surrounding and spaced from said wall means is a shell which encloses and protects the wall means forming the duct and also assists in defining additional means adapted to absorb acoustical energy. The total path of air flow through the silencer preferably includes all or a part of the annular space between the duct wall means and the shell. Both of these annularly spaced baffle means may be placed to reduce or inhibit sound transmission along flanking paths, such baflle means permitting tree air flow through the spaces in order to effect cooling of the unit.

In a preferred embodiment, increased efiectiveness is achieved by placing sound reflective means in the gas duct mounted on a sound-absorbing core centrally of the duct.

How the above objects and advantages of the invention, as well as others not specifically mentioned, are attained will be more readily understood by reference to the following description and to the annexed drawing, in which:

FIG. 1 is a longitudinal median section through a silencer, embodying the present invention, tor a turbine exhaust;

FIG. 2 is a transverse section thereof on line 22 of FIG. 1;

FIG. 3 is :a view similar to FIG. 1, showing a portion of a gas turbine engine exhaust silencer constructed of a different type of laminar sound absorber material;

FIG. 4 is a front elevational view of a piece of the laminar sound absorber material used in the modified type of silencer shown in FIG. 3;

FIG. 5 is a detailed sectional view taken on the plane indicate-d by the line 5-5 of FIG. 4;

FIG. 6 is a sectional view similar to FIG. 5 but on an enlarged scale to show certain minute features; and

FIG. 7 is a fragmentary sectional view showing a slightly modified form of the invention.

Referring now to the draw-ing, there is shown in FIG. 1 a silencer, indicated gene-rally at 10, mounted at the end of exhaust pipe 11 through which is discharged a stream of hot exhaust gases from gas turbine 12.

The silencer includes Wall means of tubular configuration with open ends in order to provide spaced gas inlet and gas outlet means, respectively, at opposite ends of the wall means. The wall means forming the gas duct comprise a first or inner layer 15 and a second or outer layer 16, both annular in cross section and having a common 3 longitudinal axis 17. Although the duct is herein described as comprising only two such layers, it will become evident from subsequent description that additional lay rs may sometimes be provided to advantage. The two layers 15 and 16 of the wall means provide a duct receiving the exhaust gas stream at one end from exhaust pipe 11 and discharging it from the other end to the atmosphere. The outer layer of the wall means surrounds and is spaced radially outward from the inner layer, thereby defining between the two layers an annular space 18 which, for reasons which will become evident, is open at each of its opposite ends. Layers 15 and 16 are shown as being of cylindrical configuration, but it will be understood that it is within the scope of the invention to use other suitable shapes; for example, one in which the diameters of the wall means increase gradually toward the outlet end thereof.

Inner wall member 15 is a permeable, laminar sound absorber, preferably of metal or a metallic substance. A laminar sound absorber is a thin, sheetlike member that is porous r fluid permeable because it is possessed of numerous small air paths through the sheet providing acoustic flow through the sheet, the sheet having a viscous flow restriction that is of the same order of magnitude as the specific acoustic impedance of the fluid, i.e., the viscous flow restriction differs from the specific acoustic impedance by less than a factor of 10. Typical materials used have a flow resistance in the vicinity of 30 to 50 rayls, but a wider range of perhaps 10 to 125 rayls is considered to be suitable. Thus, airborne sound waves are able to enter and pass through the sheet along these small air paths, losing intensity as they progress through the sheet. Metals or metallic substances are preferred for this type of service because of their structural strength, rigidity and ability to withstand the elevated temperatures encountered, but ceramics may be used under some circumstances.

Various metals or metallic substances are available for suitable laminar absorbers. More or less ideal from the standpoint of absorption of acoustical energy is a laminar absorber comprising a sheet of 'sintered, powdered metal or metal fibers indicated in FIGS. 1 and 2 by stippling on walls 15 and 16. Either of these metallic substances produces a permeable sheet having numerous air passages of a diameter approximating a few thousandths of an inch. Such a product made from sintered metal fiber, often referred to as fibe-rmetal, is now available commercially under the trade name Feltmetal. However, it may be desired for practical reasons to use a laminar absorber made from a metal sheet having a large number of very fine slits, such as that described in my copending application Serial No. 178,117, filed March 7, 1962, for Permeable Sheet Metal and Method of Making Same, now abandoned, which discloses a permeable sheet of steel or other suitable metal having numerous fine slits through it. A gas turbine exhaust silencer formed of such material is, in part, shown in FIG. 3. FIGS. 4, 5 and 6 show details of the laminar sound absorber material used in the silencer illustrated in FIG. 3, the slits being designated by numeral 19. The enlarged sectional view in FIG. 6 shows that the slits 19 have rounded edges at each side of the sheet which provides a Venturilike cross section with a restricted throat and an improved sound attenuating characteristic. The slits have a width preferably not in excess of .010 inch and typically in the range of .003 to .006 inch in width, as may be determined by the desired viscous flow resistance. In this case the sheet is made from a stainless steel or other suitable alloy in order to resist corrosion at normal operating temperatures.

The single laminar absorber 15, backed up by the surrounding air space 18, is effective as a sound absorber, and this is true even though the outer wall member 16 is a solid sheet. However, greater sound absorption can be secured by also making member 16 a laminar absorber of the same character as the inner Wall member 15, as indicated by stipplinlg in FIGS. 1 and 2, since the outer wall member, as will become apparent, is also backed up by an air space which in cooperation with a permeable laminar absorber is effective to absorb acoustical energy.

Inner wall member 15 is of larger diameter than pipe 11 and is annularly spaced therefrom at 20 to provide aspiration means introducing ambient air into the stream of exhaust gases to cool the latter by mixing with them. Preferably, such ambient air is obtained from space 18 since the aspiration means thereby induces a flow of cooling air through space 18 between the two wall members 15 and 16, such air flow being counter-current to the direction of the exhaust stream inside the inner wall layer. The flow of cooling air over the outside of wall layer 15 cools this wall by direct contact therewith, thus removing heat which passes through the metallic wall member 15 by conduction. The wall member 15 being porous, cooling air can pass through the wall from space 18 into the stream of exhaust gases and thus accomplish a. certain amount of cooling by transpiration. Although passing ambient air through space 18 raises its temperature above atmospheric, such air is still much cooler than the exhaust stream and is therefore eifective to cool exhaust gases upon mixing with them. To eifect air flow through the annular passage 18, the aspiration passage 20 is placed in communication with annular space 18 and is also cut off from communication with other air sources by some suitable means, such as a inward bend 16a in wall member 16, bringing the outer wall member into contact with the exterior of tail pipe 11. When the silencer is installed permanently on a given tail pipe, the outer wall member can be suitably shaped as shown, but when the silencer is necessarily adapted for connection to various turbines, as in ground installations, an adjustable termination of wall member 16 can be accomplished by providing a flexible cuff 16b, as shown in FIG. 7, which may be formed from a fabric such as woven glass fibers. The seal thus elfected with the exterior of tail pipe 11 helps prevent escape of sound waves at the inlet end of the wall means.

Annular space 18 is preferably provided with means between wall members 15 and 16 inhibiting the transmission of sound waves axially of the silencer in order to block the flanking path otherwise open to these sound waves. Such blocking means should not prevent axial air flow, as previously mentioned, for cooling the silencer. Both of these conditions are fulfilled by providing baflle 22 in the form of a helix which offers very little opposition to air flow but blocks any straight line transmission of sound waves.

The entire double-layer wall structure described above is enclosed in an outer shell 24 which typically may be a rearward extension of the housing normally enclosing turbine 12 and accessory equipment (not shown). Shell 24 surrounds and is spaced outwardly from the tubular wall means to provide an air space 25, backing up laminar absorber walls 16 to absorb some of the sound energy passing through the wall means.

Parallel 'bafile plates 26 disposed in radial planes serve not only as structural members giving strength and rigidity to the silencer assembly, but also serve to inhibit transmission of sound waves axially in space 25. These baflle plates 26 divide the total space 25 into a number of smaller compartments that have some of the characteristics of dead air spaces and. thereby help absorb sound energy. In order to permit air to flow through space 25 from end to end thereof, baflles 26 are each provided with openings 27. The openings in successive baffles are nonaligned in order to direct air flow through space 25 over a tortuous path which improves heat removal by the air stream while eliminating line-of-sight paths over which sound can travel. It is important to note at this point that due to the limited restriction to flow caused by the passing through the various openings 27 in the baffles,

and ultimately flows out of the shell at the outlet end by entering the annular space between wall members and 16. Alternatively, cooling air can be admitted to shell 24 from the surrounding atmosphere through one or more openings in shell 24, but the arrangement shown is preferred. By placing the inlet end of space 18 in communication with space 25 within shell 24, it will be seen that an air flow path has been established in a generally rearward direction through space 25 and thence forwardly through space 18 and ultimately into the stream of exhaust gases. In this way the aspiration means at serves to induce flow of cooling air through both of the spaces 18 and 25.

Each opening 27 in a baflle may consist merely of a hole through the baflfle plate, but it is preferred to place at each opening a short length of sheet metal tubing 30 and 30a open at both ends. The short tubes serve as acoustic elements that, in combination with the volumes between bafiles, form an acoustical network able to refiect or attenuate sound waves of certain frequencies, such frequencies than is possible if all of the ducts are the same.

Within the duct and in the path of the exhaust gases there is preferably placed sound reflecting means of a character to reflect sound waves outwardly against the wall means forming the duct, and more particularly laminar absorber 15. The sound reflecting means is preferably in the form of helical vane 32 which has suflicient axial length to provide at least one turn of 360". This configuration avoids an unduly high helix angle and at the same time closes the duct to any straight line pa-th that sound waves might follow, thereby insuring that all sound waves impinge upon the reflecting means. The helix 32 is mounted on cylindrical core member 33 extending coaxially of wall members 15 and 16. As previously mentioned, core 33 may be formed of sound absorbing material indicated by stippling in FIGS. 1 and 2 of the drawing.

The direction of inclination of helical baflle 22 with respect to longitudinal axis 17 is matched with that of the helical vane 32 so that air passing through annular space 18 continues to spin in the same direction around the axis as it mixes with the gas stream, causing a minimum loss of energy at this point. For example, if the inclination of bafile 22 is such that the air within passage 18 spins in a clockwise direction around axis 17, when viewed from the inlet end of the wall means, vane 32 has an inclination such that the exhaust gases spin in the same clockwise direction.

If values of temperature and velocity of the exhaust gases in a freely expanding stream beyond exhaust pipe 11 are plotted at radially spaced points across the gas stream, it will be found that these values are maximum at axis 17. This is the case with a simplified construction omitting vane 32 and core 33. The temperature and velocity gradients within such an exhaust gas stream both produce a refraction of sound waves outwardly away from axis 17, the effect of both gradients being additive in this respect. It can be demonstrated experimentally by measuring sound intensity along lines at various angles to axis 17 that this bending of the sound waves exists. It will be found that sound intensities are greatest at angles from 20 to 60 with the axis of the gas stream, in general the angle of maximum intensity increasing with the frequency of the sound waves measured. The general sound intensity is usually most marked in the region 45 to 50 from axis 17 along a line intersecting the axis at a point shortly beyond the end of pipe 11.

This natural refraction of the sound waves in the exhaust gas stream bends the sound waves toward the sound absorber provided here in the form of concentric layers of the wall means forming the gas duct. Intensity of the sound is reduced by viscous loss as the sound passes through wall member 15, which is backed up by enclosed air space as at 18; and the viscous transmission loss is still further increased if a second absorptive layer 16 is spaced outwardly beyond the inner layer 15 and in turn backed up by an enclosed air space as at 25. Likewise, additional sound transmission loss can be obtained by providing additional spaced layers of permeable, sound absorptive material, each backed up by an air space.

Spin vane 32 changes conditions somewhat in the gas stream. Briefly, its effect is to provide a reversal in the direction of the temperature and velocity gradients of the exhaust gas near the center of the stream with the result that these gradients tend to reach peak values in an annular zone spaced radially away from axis 17. Thus a fraction of the sound is still bent or refracted outwardly as before, but an inner portion of the sound is also bent or refracted inwardly toward core 33. It is in order to absorb this sound impinging on core 33 that the latter is preferably made of sound absorptive material. Of course, some of the sound incident upon wall member 15 and core 33 is reflected therefrom onto the other of these two members where a certain amount of sound absorption takes place.

One of the particular advantages of this construction of silencer is that there is no loss in efliciency imposed upon the turbine by adding to it the silencer. Turbine efficiency would be at a comparative minimum with gas discharge through a straight pipe; and this efliciency can be improved by adding a flared diffuser section to the straight section of the exhaust pipe. This diffuser section results in some pressure recovery. A gain in efiiciency, although not as great, is obtained similarly in the present construction by allowing the free expansion of the exhaust stream from exhaust pipe 11 to take place within the duct means. The comparatively cooler secondary air entering through aspirator passage 20 acts to confine somewhat the initial stream of hot gases, even though mixing takes place. The cold gases form a boundary layer which forms an intangible diffuser section to such an extent that efficiency is improved as a result of the addition of the silencer. This more than offsets any loss of efficiency as a result of vane 32, although the latter is purposely designed with a sufficiently low helix angle that it introduces only a negligible energy loss into the gas stream.

By mixing cooler air with the gas stream and by allowing them to expand, the velocity of the exhaust gases is reduced to an acceptably low value. Since the sound energy in the gas stream increases exponentially With velocity, it is desirable to reduce the velocity at exit from the silencer to a maximum of 300 feet per second and preferably to a lower value. This value is sufficiently low that the sound created by mixing of the exhaust stream with the surrounding atmosphere does not create an undesirable intensity of sound.

The absence of a bulk fibrous body in the silencer eliminates the danger of explosion in the silencer caused by the ignition of raw fuel entering the silencer during a delayed start-up. Any fuel passing through the porous wall 15 into space 18 is protected from ignition by the porous wall in the manner of a safety lamp with an open flame. However, if any fuel passing through wall 15 is ignited in the space 18, explosion is avoided by the pressure relief aiforded to the space by the porous wall 15.

In the foregoing description it has been indicated that various changes in the detailed construction and arrangement of the component parts of the exemplary embodiment of the invention herein described may be made with out departing from the spirit and scope of the present invention. Accordingly, it is to be understood that the foregoing description is considered as being illustrative of, rather than limitative upon, the present invention.

I claim:

1. A silencer for a stream of high velocity exhaust gases from a combustion chamber, comprising:

(a) wall means of tubular shape forming a duct through which the gas stream flows, said wall means having spaced gas inlet and gas outlet means; and

(b) means associated with said Wall means to form a plurality of tubular layers with an annular sound absorbing air space therebetween, at least the inner layer being a sheet laminar absorber of permeable material having a viscous flow restriction that is of the same order of magnitude as the specific acoustic impedance of said gas.

2. A silencer for a stream of high velocity exhaust gases from a combustion chamber, comprising:

(a) wall means of tubular shape forming a duct through which the gas stream flows, said wall means having spaced gas inlet and gas outlet means;

(b) means associated with said wall means to form a plurality of tubular layers with an annular sound absorbing air space therebetween, at least the inner layer being a sheet laminar absorber of permeable material having a viscous flow restriction that is of the same order of magnitude as the specific acoustic impedance of said gas; and

(c) means causing limited air movement between two of the layers and in contact with the outside surface of said inner layer to remove heat therefrom.

3. A silencer for a stream of high velocity exhaust gases from a combustion chamber, comprising:

(a) wall means of tubular shape forming a duct through which the gas stream flows, said wall means having spaced gas inlet and gas outlet means;

(b) means associated with said wall means to form a plurality of tubular layers with an annular sound absorbing space therebetwen, at last the inner layer being a sheet laminar absorber of permeable material having a viscous flow restriction that is of the same order of magnitude as the specific acoustic impedance of said gas; and

(c) aspiration means creating restricted axial flow of ambient air between two of said layers and in contact with the outside surface of said inner layer to remove heat therefrom. I

4. A silencer for a stream of high velocity exhaust gases from a combustion chamber, comprising:

(a) wall means of tubular shape forming a duct through which the gas stream flows, said wall means having spaced gas inlet and gas outlet means; and

(b) means associated with said wall means to form a plurality of tubular layers with an annular sound absorbing space therebetween, at least the inner layer being a sheet laminar absorber of sintered metal having a viscous flow restriction that is of the same order of magnitude as the specific acoustic impedance of said gas.

5. A silencer for a stream of high velocity exhaust gases from a combustion chamber, comprising:

(a) wall means of tubular shape forming a duct through which the gas stream flows, said wall means having spaced gas inlet and gas outlet means; and

(b) means associated with said wall means to form a plurality of tubular layers with an annular sound absorbing space therebetween, at least the inner layer being a laminar absorber of sheet metal having numerous fine slit-s providing a viscous flow restriction that is of the same order of magnitude as the specific acoustic impedance of said gas.

6. A silencer for a stream of high velocity exhaust gases from a combustion chamber, comprising:

(a) wall means of tubular shape forming a duct through which the gas stream flows, said wall means having spaced gas inlet and gas outlet means; and

(b) means associated with said Wall means to form a plurality of tubular layers having a sound absorbing space therebetween, tWo successive layers of the wall means being laminar absorbers having a viscous flow restriction that is of the same order of magnitude as the specific acoustic impedance of said gas.

7. A silencer for a stream of high velocity exhaust gases from a combustion chamber, comprising:

(a) wall means of tubular shape forming a duct through which the gas stream flows, said wall means having spaced gas inlet and gas outlet means;

(b) means associated with said wall means to form a plurality of tubular layers with sound absorbing air space therebetween, each of said layers being laminar absorbers having a viscous flow restriction that is of the same order of magnitude as the specific acoustic impedance of said gas;

(c) baffle means between said tubular layers inhibiting the transmission of sound axially through said air space, said baflie means being arranged to permit limited air flow through said air space; and

(d) means for inducing the flow of ambient air into and through said air space.

8. A silencer for a stream of high velocity exhaust gases from a combustion chamber, comprising:

(a) wall means of tubular shape forming a duct through which the gas stream flows, said wall means having spaced gas inlet and gas outlet means;

(b) means associated with said wall means to form a plurality of tubular layers With an air space therebetween, each of said layers being laminar absorbers having minute air paths therethrough permitting airborne sound waves to enter and pass through the layer with diminished intensity;

(c) baffle means between said tubular layers inhibiting the transmission of sound axially through said air space, said baflle means being arranged to permit limited air flow through said air space;

(d) a shell around and spaced radially from said wall means; and

(e) baflie means within said shell inhibiting the propagation of sound waves between the shell and the wall means axially of the silencer, said baflie means being disposed to permit air flow through the space between the shell and wall means.

9. A silencer for a stream of high velocity exhaust gases from a combustion chamber, comprising:

(a) wall means of tubular configuration forming a duct through which the gas stream flows, said wall means having spaced inlet and outlet means, said wall means comprising an inner layer that is a metallic sheet laminar absorber of permeable material having a viscous flow restriction that is of the same order of magnitude as the specific acoustic impedance of said gas; and

(b) sound reflecting means inside the duct means in the path of said stream of gases to reflect sound Waves outwardly against the Wall means.

10. A silencer for a stream of high velocity exhaust gases from a combustion chamber, comprising:

(a) Wall means of tubular configuration, forming a duct through which the gas stream flows, said wall means having spaced inlet and outlet means;

(b) sound reflecting means inside said duct in the path of said stream of gases to reflect sound waves outwardly against said wall means; and

(c) a second wall means of tubular configuration surrounding said first-mentioned wall means and spaced radially outwardly therefrom to provide a sound absorbing space between said first-mentioned and said second wall means, both of said wall means comprising metallic sheet laminar absorbers of permeable material having a viscous flow restriction that is of the same order of magnitude as the specific acoustic impedance of said gas.

11. A silencer for a stream of high velocity exhaust gases from a combustion chamber, comprising:

(a) wall means of tubular configuration forming a duct through which the gas stream flows, said wall means having spaced inlet and outlet means;

(b) sound reflecting means inside said duct in the path of said stream of gases to reflect sound waves outwardly against said wall means;

(c) a Second wall means of tubular configuration surrounding said first-mentioued wall means and spaced radially outwardly therefrom to provide a substantially dead air space between said first-mentioned and said second wall means, both of said wall means comprising metallic sheet laminar absorbers of permeable material having air paths with at least one transverse dimension not exceeding .010 inch;

(d) means for inducing a flow of ambient air into and through the space between said Wall means and in contact with the outer surface of the inner wall means; and

(e) means in the space between said wall means to impede the transmission of sound axially of the silencer, said sound impeding means being arranged to permit limited air flow through said space.

12. A silencer for a stream of high velocity exhaust gases from a combustion chamber, comprising:

(a) wall means of tubular configuration forming a duct through which the gas stream flows, said wall means having spaced inlet and outlet means;

I (b) sound reflecting means inside said duct in the path of said stream of gases to reflect sound waves outwardly against said wall means;

a (c) a second wall means of tubular configuration surrounding said first-mentioned wa'll means and spaced radially outwardly therefrom to provide a substantially dead air space between said first-mentioned and said second wall means, both of said wa-ll means comprising metallic sheet laminar absorbers of permeable material having air paths with at least one transverse dimension not exceeding .010 inch;

((1) means in the space between said wall means to impede the transmission of sound axially of the silencer, said sound impeding means being arranged to permit limited air flow through said space; and

(e) aspirator means at the inlet end of said duct to induce a flow of air from the space between said wall means into the gas stream in said duct, said space being open to admit ambient air thereto at a location near the outlet end of said duct.

13. A silencer for a stream of high velocity exhaust gases from a combustion chamber, comprising:

(a) wall means of tubular configuration forming a duct through which the gas stream flows, said wall means having spaced inlet and outlet means, said wall means comprising an inner 'layer that is a metallic sheet laminar absorber of permeable material having a viscous flow restriction that is of the same order of magnitude as the specific acoustic impedance of said gas; and

(b) a helical spin vane disposed coaxially in said duct in the path of said stream of gases to reflect sound waves outwardly against said Wall means.

14. A silencer for a stream of high velocity exhaust gases from a combustion chamber, comprising:

(a) wall means of tubular configuration forming a duct through which the gas stream flows, said wall means having spaced inlet and outlet means, said wall means comprising an inner layer that is a metallic sheet laminar absorber of permeable material having a viscous flow restriction that is of the same order of magnitude as the specific acoustic impedance of said gas.

(b) sound reflecting means inside the duct means 1n the path of said stream of gases to reflect sound waves outwardly against the wall means;

(c) a shell around and spaced radially from the wall means to provide an annular air space therearound; and

(d) baflie means within said shell to inhibit propagation of sound waves axially of the space between the shell and wall means.

15. A silencer for a stream of high velocity exhaust gases from a combustion chamber, comprising:

(a) wall means of tubular configuration forming a duct through which the gas stream flows, said wal-l means having spaced inlet and outlet means, said wall means comprising an inner layer that is a metallic sheet laminar absorber of permeable material having an paths with at least one transverse dimension not exceeding .010 inch through the layer permitting airborne sound waves to enter and pass through the layer with diminished intensity;

(b) sound reflecting means inside the duct means in the path of said stream of gases to reflect sound waves outwardly against the wall means;

(0) a shell around and spaced radially from the wall means to provide an annular air space therearound;

(d) baflie means provided with a plurality of openings within said shell inhibiting the propagation of sound waves axially within the space 'between the shell and wall means; and

(e) tuned open-ended ducts located at the openings in said baflie means to attenuate sound waves of predetermined frequencies.

16. A silencer for a stream of high velocity exhaust gases from a combustion chamber, comprising:

(a) wall means of tubular configuration forming a duct through which the gas stream flows, said wall means having spaced inlet and outlet means, said wall means comprising an inner layer that is a metallic sheet laminar absorber of permeable material having air paths with at least one transverse dimension not exceeding .010 inch through the layer permitting airborne sound waves to enter and pass through the layer with diminished intensity;

(b) sound reflecting means inside the duct means in the path of said stream of gases to reflect sound waves outwardly against the wall means;

(c) a shell around and spaced radially from the wall means to provide an annular air space therearound;

(d) baflle means provided with a plurality of openings within said shell, said openings being arranged in staggered relationship to cause said baflle means to inhibit the propagation of sound waves axially within the space between the shell and wall means; and

(e) means inducing limited air flow through the space between the shell and the wall means and into the gas stream.

17. A silencer for a stream of high velocity exhaust gases from a combustion chamber, comprising:

(a) wall means of tubular configuration forming a duct through which the gas stream flows, said wall means having spaced inlet and outlet means, said wall means comprising an inner layer that is a metallic sheet laminar absorber of permeable material having a viscous flow restriction that is of the same order of magnitude as the specific acoustic impedance of said (b) a core of sound absorbent material extending centrally within and coaxially of said duct; and

(c) sound reflecting means around said core inside said duct in the path of the stream of gases to reflect sound waves outwardly against said wall means.

'18. A silencer for a stream of high velocity exhaust gases from a combustion chamber, comprising:

(a) a first wall of tubular configuration forming a duct surrounding and confining a gas stream flowing axially therethrough, said wall means having axially spaced gas inlet and gas outlet means;

(b) a second wall around and annularly spaced from the first wall, both of said walls being formed from sheets of permeable metallic material having a viscous flow restriction that is of the same order of magnitude as the specific acoustic impedance of said aspiration means inducing a flow of ambient air into and through the space between the first and second walls in a direction opposed to the flow in said duct and thence into the duct and the gas stream at the upstream end of the duct; and

(d) a shell around and radially spaced from the second wall.

19. A silencer for a stream of high velocity exhaust gases from a combustion chamber, comprising:

(a) a first wall of tubular configuration forming a duct surrounding and confining a gas stream flowing axially therethrough, said wall means having axially spaced gas inlet and gas outlet means;

(b) a second wall around and radially spaced from said first wall, both of said walls being formed from sheets of'permeable metallic material having internal air paths with at least one transverse dimension under .010 inch permitting airborne sound waves to enter and pass through the wall with diminished intensity;

(c) a shell around and radially spaced from the second wall, the space between the first and second wall communicating with the space between the shell and second wall near the downstream end of the silencer to permit ambient air flow through said two spaces in series; and

(d) aspiration means inducing a flow of ambient air into and through the space between the first and second walls in a direction opposed to the flow in said duct and thence into the duct and the gas stream at the upstream end of the duct.

20. A silencer for a stream of high velocity exhaust gases from a combustion chamber, comprising:

(a) wall means of tubular configuration forming a duct through which the gas stream flows, said wall means having spaced inlet and outlet means, said wall means comprising an inner layer that is a metallic sheet laminar absorber of permeable material having a viscous flow restriction that is of the same order of magnitude as the specific acoustic impedance of said 8 (b) sound reflecting means inside the duct means in the path of said stream of gases to reflect sound waves outwardly against the wall means;

(0) a second wall means around and spaced radially from said first-mentioned Wall means to provide an annular air space therearound;

(d) a shell around and spaced radially from said second wall means to provide an annular air space therearound; and

(e) baffle means in the spaces between said first and second wall means and between said second wall means and said shell, said baflle means inhibiting the propagation of sound waves within the respective spaces, said baffie means being .arranged to permit limited air flow through such spaces.

21. A silencer for a stream of high velocity exhaust gases from a combustion chamber, comprising:

(a) a first wall of tubular configuration forming .a duct surrounding and confining a gas stream flowing axially therethrough, said wall means having axially spaced gas inlet and gas outlet means;

(b) a second wall around and annularly spaced from the first Wall, both of said Walls being formed from sheets of permeable metallic material having internal air paths with at least one transverse dimension under .010 inch permitting airborne sound waves to enter and pass through the wall with diminished intensity;

(c) a shell around and spaced radially from the second wall to provide an annular air space therearound, the latter air space communicating with the air space between the first and second wall near the downstream end of the silencer to permit ambient air to flow through the air spaces in series; and

(d) a flexible cutf member mounted on said second Wall and forming a reverse bend extending around and into the gas inlet means of the first wall.

22. A silencer for a stream of high velocity exhaust gases from a combustion chamber, comprising:

(a) a first wall of tubular configuration forming a duct surrounding and confining a gas stream flowing axially therethrough, said wall means having axially spaced gas inlet and gas outlet means;

(b) a second wall around and annularly spaced from the first wall, both of said walls being formed from sheets of permeable metallic material having internal air paths with at least one transverse dimension under .010 inch permitting airborne sound waves to enter and pass through the wall with diminished intensity;

(c) a shell around and spaced radially from the second wall to provide an annular air space therearound, the latter air space communicating with the air space between the first and second wall near the downstream end of the silencer to permit ambient air to flow through the air spaces in series;

(d) a inward bend on said second wall extending around and into the gas inlet means of the first wall; and

(e) baffle means in the spaces between said first and second walls and between the second wall and said shell, said baflle means inhibiting the propagation of sound waves within the respective spaces, said bafile means being arranged to permit limited air flow through such spaces and into the gas stream in said duct.

References Cited by the Examiner UNITED STATES PATENTS 851,490 4/1907 Brisben 181-55 X 1,854,830 4/1932 Flanders. 1,918,149 7/1933 Sullivan. 1,991,206 2/1935 Harrison. 2,050,581 8/1936 Orem 181-55 X 2,115,128 4/1938 Starkweather et al. 2,116,751 5/1938 Dererner 18l54 2,118,056 5/1938 Peik. 2,595,047 4/ 1952 Beranek. 7 2,810,449 10/ 1957 Coleman 18143 2,944,623 7/ 1960 Bodine. 2,950,775 8/1960 Zwayer 181-36 2,988,302 6/1961 Smith 181--51 X 3,185,252 5/1965 Lemmerman 18151 FOREIGN PATENTS 564,474 11/1932 Germany. 358,970 10/1931 Great Britain. 829,012 2/1960 Great Britain. 831,776 3 /1960 Great Britain.

RICHARD B. WILKINSON, Primary Examiner.

LOUIS I. CAPOZI, Examiner. ROBERT S. WARD, Assistant Examiner. 

1. A SILENCER FOR A STREAM OF HIGH VELOCITY EXHAUST GASES FROM A COMBINATION CHAMBER, COMPRISING: (A) WALL MEANS OF TUBULAR SHAPE CORMING A DUCT THROUGH WHICH THE GAS STREAM FLOWS, SAID WALL MEANS HAVING SPACED GAS INLET AND GAS OUTLET MEANS; AND (B) MEANS ASSOCIATED WITH SAID WALLS MEANS TO FORM A PLURALITY OF TUBULAR LAYERS WITH AN ANNULAR SOUND ABSORBING AIR SPACE THEREBETWEEN, AT LEAST THE INNER 